The present invention relates to an angular rate sensor.
FIG. 3 shows an example of angular rate sensors proposed in the past. In FIG. 3, a support pin 101 made of metal is press-fitted perpendicularly and secured in a weight plate (not shown in the figure), and one end of another support pin 102 also made of metal is press-fitted and secured in the support pin 101 in an orthogonal direction to it. A block 103 made of metal is fixed by soldering at the other end of the support pin 102, which also serves as a common terminal. Vibration plates 104 and 105 are fixed at both ends of the metal block 103. A piezoelectric element 106 is bonded on the vibration plate 104 to constitute a vibration exciter 150, and another piezoelectric element 107 is bonded on the vibration plate 105 to constitute a means 160 for detecting a level of vibrations. A tip of the vibration plate 104 is extended in a manner to form a right angle with the piezoelectric element 106 to become a detecting plate 108. A tip of the vibration plate 105 is also extended in the same manner to form a right angle with the piezoelectric element 107 to become another detecting plate 109. Piezoelectric elements 110 and 111 are bonded respectively on the detecting plates 108 and 109, to constitute detecting means 170 and 180 for detecting a Coriolis"" force generated in proportion to an angular rate. All of the above complete an element unit 112 of a tuning-fork type angular rate sensor.
A structure of FIG. 3 further comprises;
(a) a current amplifier circuit 120 for amplifying an output signal from the piezoelectric element 107 provided on the vibration plate 105 to detect a level of vibrations of the vibration plate 105, which vibrates in a tuning-fork phenomenon in concert with vibrations of the vibration plate 104;
(b) a full-wave rectifier circuit 122 for producing a D.C. voltage by rectifying an output signal (i.e. a signal at a point xe2x80x9cAxe2x80x9d, of which a signal voltage waveform is shown in FIG. 4) of a band-pass filter circuit (hereinafter referred to as xe2x80x9cBPF circuitxe2x80x9d) 121, wherein an output signal of the current amplifier circuit 120 is input;
(c) an automatic gain control circuit (hereinafter referred to as xe2x80x9cAGCxe2x80x9d) 123 whose amplification factor for the output signal of the BPF circuit 121 varies according to a magnitude of an output signal of the full-wave rectifier circuit 122;
(d) a driver circuit 124 (an output signal of this circuit, i.e. a signal at a point xe2x80x9cBxe2x80x9d, has a voltage waveform shown in FIG. 4) for driving the piezoelectric element 106 bonded on the vibration plate 104 according to a magnitude of an output signal of the AGC 123;
(e) a charge amplifier circuit 125 for inputting and amplifying output signals of the piezoelectric elements 110 and 111, which detect a Coriolis"" force generated in proportion to an angular rate;
(f) a synchronous detection circuit 127 for detecting an output signal of a BPF circuit 126, wherein an output signal of the charge amplifier circuit 125 is input; and
(g) a sensor output terminal 129 for outputting an output signal of a low-pass filter circuit (hereinafter referred to as xe2x80x9cLPF circuitxe2x80x9d) 128, wherein an output signal of the synchronous detection circuit 127 is input.
In addition, a reference voltage generating means 132 comprises a power supply 130 and a buffer 131. The reference voltage generating means 132 supplies a reference voltage to each of the above-cited circuits through a circuit resistance 133 (let a resistance value be xe2x80x9cR1xe2x80x9d).
A terminal 135 is also provided for connecting the reference voltage generating means 132 to the support pin 102 via the circuit resistance 133 and another circuit resistance 134 (let a resistance value be xe2x80x9cR2xe2x80x9d). The foregoing elements constitute a driving circuit 136.
The element unit 112 of a tuning-fork type angular rate sensor and the driving circuit 136 complete the angular rate sensor.
In the prior art technique, an alternate current xe2x80x9cixe2x80x9d flows from the driver circuit 124 toward the reference voltage generating means 132 via the terminal 135 by passing through the vibration exciter 150 at all the time, even in an ordinary vibrating condition of the tuning fork.
In addition, a demand for reduction in size of the angular rate sensors necessitates an integration of the driving circuit 136 into an IC tip form. This consequently reduces a width of wiring pattern, which in turn increases resistance values of the individual circuit resistances 133 and 134.
Ripple voltage of a large magnitude defined by (R1+R2)xc2x7i is therefore generated between the circuit resistances 133 and 134 (this ripple voltage is observed at a point xe2x80x9cCxe2x80x9d, and a waveform of the signal voltage is shown in FIG. 4).
The ripple voltage subsequently causes a substantial difference between the reference voltage input to individual circuits and the voltage at the terminal 135. A displacement current flows as a result (this displacement current is observed at a point xe2x80x9cDxe2x80x9d, of which a signal current waveform is shown in FIG. 4) from the piezoelectric elements 110 and 111. This displacement current is input in the charge amplifier circuit 125, and an output signal voltage of it appears at a point xe2x80x9cExe2x80x9d (a waveform of the signal voltage is shown in FIG. 4). However, this signal voltage turns into an output signal of the synchronous detection circuit 127 and appears at a point xe2x80x9cFxe2x80x9d (a waveform of this signal voltage is shown in FIG. 4) without being cut off in a process of synchronous detection, since it is in a same phase as the waveform of the signal voltage at the point xe2x80x9cAxe2x80x9d, i.e. a timing signal, of the synchronous detection circuit 127. This output signal eventually becomes an offset voltage (this offset voltage is observed at a point xe2x80x9cGxe2x80x9d, as shown in FIG. 4), and it comes out at the output terminal 129. This offset voltage denoted as xcex94V is given by a formula (1):
xcex94V=Axc2x7Dxc2x7(R1+R2)xc2x7ixc2x7(1/C0)xc2x7(Cs1+Cs2)xc2x7sin xcfx86xe2x80x83xe2x80x83(1),
where:
A is a gain of the low-pass filter and the band-pass filter;
D is a detection coefficient;
C0 is a feedback capacity of the charge amplifier, in pF; and
Cs1 and Cs2 are electrostatic capacities of the piezoelectric elements 110 and 111, in pF.
In addition, it is likely that a variation occurs with the reference voltage input to the individual circuits, since ripple voltage of a large magnitude defined by R1xc2x7i is generated in the circuit resistance 133.
An angular rate sensor of the present invention comprises:
(a) a vibration exciter for providing a vibration body with vibrations;
(b) a means for detecting a level of vibrations of the vibration body;
(c) a detecting means for detecting a Coriolis"" force produced in proportion to an angular rate;
(d) a current amplifier circuit for amplifying an output signal of the means of detecting a level of vibrations;
(e) a full-wave rectifier circuit for producing a D.C. voltage by rectifying an output signal of a band-pass filter circuit, wherein an output signal of the current amplifier circuit is input;
(f) an automatic gain control circuit whose amplification factor for the output signal of the band-pass filter circuit varies according to a magnitude of an output signal of the full-wave rectifier circuit;
(g) a driver circuit for driving the vibration exciter in accordance with a magnitude of an output signal of the automatic gain control circuit;
(h) a charge amplifier circuit for inputting and amplifying a signal detected by the detecting means for detecting a Coriolis"" force;
(i) a synchronous detection circuit for detecting an output signal of a band-pass filter circuit, wherein an output signal of the charge amplifier circuit is input;
(j) a sensor output terminal for outputting an output signal of a low-pass filter circuit, wherein an output signal of the synchronous detection circuit is input;
(k) the reference voltage generating means; and
(l) a buffer provided between the reference voltage generating means and the common terminal for the vibration exciter, the means for detecting a level of vibrations and the detecting means for detecting a Coriolis"" force, for suppressing a current flowing in and out of the common terminal so as to avoid it from flowing in and out of a side of the reference voltage generating means. This structure can realize the angular rate sensor capable of restraining the output voltage of the sensor from being offset.